How Pluto's Newfound Moon Was Discovered

Searching for a ring around the Pluto, scientists accidentally discovered that the former ninth planet has a tiny fourth moon. The satellite, temporarily named P4, will be added to the list of things that NASA's New Horizons spacecraft examines when it reaches the system in 2015.

Using the Hubble space telescope, scientists have discovered that Pluto has a fourth moon, NASA announced today. Temporarily dubbed P4, the tiny satellite is estimated to be only eight to 25 miles in diameter. Johns Hopkins University researcher Hal Weaver, part of the team doing the observations, says far-away Pluto (itself about 1400 miles wide) appears 30,000 times brighter than the tiny moon, which is how P4 escaped discovery until now. "It's like looking for a firefly in front of a headlight," he says.

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The Hubble team that found P4 was originally looking to see if Pluto has a ring. They were trying to find a path to guide the New Horizons spacecraft, a NASA mission due to reach the dwarf planet in 2015, and such a ring of debris could present an obstacle. Team leader Mark Showalter, an astronomer with the SETI Institute in Mountain View, Calif., says the researchers found no ring—good news for New Horizons—and that P4's discovery makes the mission even more exciting. "The Pluto system is a much more interesting and dynamic system than we realized before," he says.

Showalter and his team aimed Hubble at Pluto for three sets of observations during the last month to confirm the find. To see the faint signs of a dust ring (or the tiny satellite they accidentally spotted), the scientists took more sensitive measurements of Pluto than any taken before, using an exposure length of 8 minutes so Hubble could gather as much light as possible. Weaver and other researchers then studied observations of Pluto from 2006 and 2010, and their careful analysis found tale-tale signs of P4 in that data as well. The researchers didn't catch P4 in 2006 because it was obscured by the diffraction spike, the X-shaped pattern caused by Pluto's brightness. (It's the same kind of X-shaped light you've seen if you've ever tried to take a picture of a bright light or snapped a flash photo in front of a reflective surface).

Researchers think P4 lies between Hydra, Pluto's outermost moon, and Nix, which were both discovered in 2005 and thought to be between 20 to 70 miles wide. Charon is the planet's brightest and largest satellite, about 650 miles across. Showalter's team will soon decide on a name for the moon, which, in keeping with Pluto's other satellites, will probably come from a Greek god of the underworld.

The genesis of Pluto's several satellites probably happened billions of years ago. Researchers think that Pluto smacked into a similarly sized object about 4.6 billion years ago, tearing off tons of surface material that later coalesced into several satellites, Showalter says. It happened around the same time an impact between Earth and a Mars-sized object formed our planet's moon, scientists hypothesize. Alan Stern, principal investigator of the New Horizons mission to Pluto, says the craft will be able to measure the composition of Pluto and its satellites, which will give researchers a better idea how the system formed. "When we fly by we will re-write the book on Pluto," Weaver says.

However, Showalter says, the finding of a fourth moon is unlikely to change the controversial designation of Pluto as a dwarf planet rather than a planet, which happened with the International Astronomical Union's ruling back in 2006. "But I do think it gives it more respect," Weaver says.